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According to the recordings of the DFDR etc., it took approximately three minutes for
cabin pressure to be virtually equalized with ambient pressure. According to the
classification of decompression in AC61-107, this event is recognized as a Gradual (slow)
Decompression.
3.1.5 At the first 4C Check visit of the said aircraft, the gates of safety valves installed up
to that time had not opened even at the maximum correct positive differential pressure. The
safety valves were therefore replaced with S/N127 and S/N129 that had been stored as
spare parts. At this point, as the AMM did not specify confirmation of the differential
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pressure function, a functional test of the gate opening of said valves was not carried out.
3.1.6 After the first 4C Check visit, the said aircraft made three flights before this serious
incident occurred. It is estimated that the maximum cruising altitude reached during this
period was 25,000ft, and that the maximum differential pressure between the inside and
the outside of the cabin was 7.37psi. Further, it is estimated that during the flight in which
this serious incident occurred, the said aircraft was cruising at an altitude of around
35,000ft, and the differential pressure between the inside and the outside of the cabin was
around 8.03psi.
3.1.7 Based on the recordings of the DFDR etc., while the said aircraft was in cruise the
cabin altitude increased abruptly from around 6,600ft to around 21,600ft. Following the
descent of the said aircraft, after 11:56 the cabin altitude and the aircraft altitude
maintained a constant differential, with the cabin altitude slightly lower than the flight
altitude, and the rates of change of flight altitude and cabin altitude were virtually constant.
It is considered that this is because the gate of a safety valve did not close after having
opened. As described below, as a result of anomalies found in safety valve S/N129, it is
estimated that the cause was the incorrect operation of safety valve S/N129.
(1) As the result of the investigation of the cabin pressurization system of the said
aircraft as described in section 2.8.3 and the functional test of the safety valve as
described subsection 2.8.4(1)②, it was found that the one of the set of two safety
valves that had been installed, S/N129, opened at below the specified differential
pressure limit. Therefore, it is estimated the pressurized air from the aircraft cabin
had leaked through the gate of the safety valve S/N129.
(2) Based on the functional test of the safety valve as described subsection 2.8.4(1)③,
because the abnormal behavior was transferred from safety valve S/N129 to safety
valve S/N127, it is estimated that there were abnormalities in the stem assembly of
safety valve S/N129.
(3) From the fact that there was no maintenance history indicating disassembly of the
said valve prior to the occurrence of this serious incident, it is estimated that the
anomalies—viz. the abnormalities existing inside the stem assembly (the existence of
contamination and burrs at the edges of a hole, and the bush hole being of a smaller
diameter than the specified limit), and that a thicker shim than specified had been
used—had already been present at the manufacturing stage. Further, it is estimated
that the increase in the preload of spring-“A” of the control valve during the
adjustment work carried out at the ARS resulted in the gate opening at a value below
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the specified limit.
(4) A higher cruise altitude had been selected for the flight concerned than had been
flown in the other three flights since the 4C Check in which the safety valves had
been replaced. It is considered possible that during cruise, changes in the
environment due to vibrations during flight etc. resulted in the control valve
operating (due to motion of the stem poppet, which was stuck to the seat housing) at
a lower pressure differential than the limit that had been set, which caused the gate
to open. It is further estimated that as a result of the abnormalities of the stem
assembly described in (3) above, the motion of the stem poppet was restricted and as
a result the open condition remained until the time of landing.
(5) The gate of the safety valve first opened at FL350 (differential pressure 8.03psi), it
opened at 5.16psi at the time of investigation of the cabin pressurization system of
the aircraft, and at the time of the detailed investigation of the safety valves a
differential pressure above 0.39psi could not be supported. The function was thus
getting clearly worse. Furthermore, when the differential pressure functional test
was carried out after swapping the safety valve of S/N129 with that of S/N127, a
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AIRCRAFT SERIOUS INCIDENT INVESTIGATION REPORT ALL NIPPON AI(8)
